This invention relates to method and apparatus for forming a double-layer hollow film, in which a pair of film layers are interconnected integrally by a number of spaced legs so as to define a number of elongated compartments between the pair of film layers.
A hollow film of this type in the excellent heat insulative property, i.e. heat-storing property, compared with a conventional mono-layer sheet film, and therefore has recently drawn much attention as a covering material for greenhouses. However, if such a hollow film has extremely lower light permeability than the conventional mono-layer sheet film because of its double-layer hollow construction, solar heat in the daytime cannot be stored sufficiently within the greenhouse. Further, if a number of legs bridging between the pair of film layers cannot separate these films sufficiently to define elongated compartments therebetween, the heat insulation effect is decreased too low to be used as covering material a greenhouse. In addition, it is required that such a hollow film can be wound up for storage transportation as in the case of the conventional mono-layer sheet film. It is also necessary for the hollow film to have sufficient flexibility to increase working efficiency.
In the conventional technique, however, it has been impossible to economically form such double-layer hollow film having a pair of film layers integrally molded with a plurality of legs from a transparent, thin and flexible for obtaining a desired level of light permeability with the legs being thin but firm to prevent the compartments defined thereby between the pair of film layers from being deformed or crushed.
In one method for forming a double-layer hollow film, an inflation-extrusion process has been proposed as disclosed in Japanese Patent Publication No. 133263/1975. According to this inflation-extrusion process, a molten thermoplastic resin is extruded and inflated outwardly of an annular extruding die including a pair of annular slits, having different diameters and arranged concentrically, and a plurality of slits interconnecting between these annular slits. Upon inflating the thus extruded resin, in order to prevent the legs interconnecting the film layers from being deformed irregularly or bent, it becomes necessary to regulate the pressure of gas supplied to the center space of the annularly inflated film and also the pressure of gas supplied inbetween the smaller and larger diameter slits. However, such regulation is extremely difficult to carry out by present day techniques. Further, in the known process, the inflated resin is subjected to air-cooling for curing. However, air-cooling takes a considerate amount of time, during which the resin is generally likely to be stretched or extended to be so thin that the legs between the film layers are distorted or bent. The light permeability of the resin is also decreased during a slow cooling operation, resulting in low utility.
Water-cooling can be employed instead of air-cooling. The conventional water-cooling process, however, which has been used for cooling mono-layer film, cannot provide a simultaneous and uniform cooling to the two film layers without deforming the compartments defined therebetween. Thus, water-cooling has the defects as in the case of the air-cooling.
It is therefore an object of the present invention to improve the above-mentioned known method and apparatus for forming a double-layer hollow film.
Another object of the present invention is to provide a method and apparatus for forming a double-layer hollow film having high light permeability and desired thickness as well as flexibility.
A further object of the present invention is to provide a method and apparatus as set forth above in which the double film layers are separated as desired by legs therebetween for enhancing heat insulation.